Object information acquiring apparatuses that measure functional characteristics inside an object such as body tissue include known object information acquiring apparatuses which perform determination of formation of angiogenesis, calculation of oxygen saturation of hemoglobin, and the like based on light-absorbing characteristics of hemoglobin or the like contained in blood and which are utilized in diagnosis. Such apparatuses generally use near-infrared light (with a wavelength of around 600 to 1500 nm) having favorable light transmission characteristics with respect to body tissue.
With such object information acquiring apparatuses, there is a known technique which uses near-infrared light and which is referred to as Photoacoustic Tomography (PAT) (refer to NPL 1). PAT is a technique for acquiring information related to light absorption by irradiating short pulse light generated by a light source on a living organism and detecting acoustic waves generated when light propagated and diffused in the living organism is absorbed by body tissue such as blood. By analyzing detected acoustic waves, an object information acquiring apparatus utilizing PAT is capable of visualizing information related to functional characteristics inside a living organism that is an object and, in particular, a light energy absorption density distribution inside the object.
According to NPL 1, in PAT, initial sound pressure (P0) of a photoacoustic wave generated by a light absorber inside a given object can be expressed by equation (1) below.P0=Γ·μa·φ  (1)
In equation (1), Γ denotes a Gruneisen parameter that is a quotient of a product of a coefficient of volumetric expansion (β) and a square of the speed of sound (c) divided by specific heat under constant pressure (Cp). Γ is known to assume an approximately constant value once tissue is determined. In the case of breast tissue, Γ is 0.65 to 0.85. μa denotes an absorption coefficient of an absorber, and φ denotes an amount of light in a localized region.
A time variation of sound pressure P of an acoustic wave having propagated inside an object is calculated by measuring the acoustic wave with a detector and subsequently reconstructing an initial sound pressure distribution from a result of the measurement. In addition, by dividing the calculated initial sound pressure distribution by Γ, a distribution with respect to μa and φ can be obtained. The product of μa and φ is referred to as a light energy absorption density distribution.
When the object is a living organism, tissue that efficiently absorbs near-infrared light is blood. Therefore, by performing measurement of a living organism with PAT using near-infrared light, information related to blood distribution is obtained. Furthermore, by irradiating light of a plurality of wavelengths and calculating respective absorption coefficients thereof, information related to oxygen saturation of hemoglobin is obtained.
As expressed by equation (1), general PAT is designed to obtain a distribution of initial sound pressure P0 by analyzing a time variation of sound pressure as measured by an acoustic wave detector. A distribution of amount of light inside the object must be further determined in order to obtain a distribution of light absorption coefficients from the distribution of initial sound pressure P0.
When a surface of a living organism that is an object is irradiated with a uniform amount of irradiated light φ0 in a region that is sufficiently large with respect to a thickness of the living organism, assuming that light propagates in the living organism as a planar wave, a distribution of the amount of light (φ) can be expressed by equation (2) below.φ=φ0·exp(−μeff·d1)  (2)
In equation (2), μeff denotes an average effective delay coefficient of the living organism and φ0 denotes an amount of light incident to the living organism from the light source. In addition, d1 denotes a distance from the region (light-irradiated region) on the living organism which is irradiated by light from the light source to a light absorber in the living organism.
As expressed by equation (2), light decays exponentially inside the living organism. A light absorption coefficient distribution can be calculated from a light energy absorption density distribution using the distribution of the amount of light and equation (1).
In addition, with body tissue, formation of angiogenesis and an increase in oxygen consumption are known to occur during growth of a tumor such as cancer. Light absorption coefficients of oxyhemoglobin (HbO2) and deoxyhemoglobin (Hb) can be used as a method of evaluating such a formation of angiogenesis or increase in oxygen consumption.
For example, an object information acquiring apparatus measures concentrations of HbO2 and Hb in blood based on absorption spectra of HbO2 and Hb at a plurality of wavelengths. Subsequently, by creating concentration distribution images of HbO2 and Hb in body tissue, a region in which angiogenesis are formed can be determined. In addition, by calculating oxygen saturation based on the concentrations of HbO2 and Hb, a region in which oxygen consumption has increased or, in other words, a region in which a tumor conceivably exists can be determined. For example, it is known that oxygen saturation in a vein is around 90% and oxygen saturation in a tumor region is around 60%.
However, with the object information acquiring apparatus described in NPL 1, since a shape of a living organism that is an object is not determined and light irradiating conditions vary depending on a position of the object, a distribution of the amount of light inside the object cannot be calculated using equation (2).
In consideration thereof, PTL 1 proposes a living organism information processing method in which a light-absorbing member whose functional characteristics with respect to irradiated light are known in advance is arranged on an object holding plate and a light decay coefficient inside the object is calculated based on an intensity of an elastic wave that is generated by the light-absorbing member.